Light emitting diodes (LEDs) are an important class of solid state devices that convert electric energy to light and generally comprise an active layer of semiconductor material sandwiched between two oppositely doped layers. When a bias is applied across the doped layers, holes and electrons are injected into the active layer where they recombine to generate light.
There has been a great deal of recent interest in LEDs formed of Group-III nitride based material systems because of their unique combination of material characteristics including high breakdown fields, wide bandgaps (3.36 eV for GaN at room temperature), large conduction band offset, and high saturated electron drift velocity. The doped and active layers are typically formed on a substrate that can be made of different materials such as silicon (Si), silicon carbide (SiC), and sapphire (Al2O3). SiC wafers are often preferred because they have a much closer crystal lattice match to Group-III nitrides, which results in Group III nitride films of higher quality. SiC also has a very high thermal conductivity so that the total output power of Group III nitride devices on SiC is not limited by the thermal resistance of the wafer (as is the case with some devices formed on sapphire or Si). Also, the availability of semi insulating SiC wafers provides the capacity for device isolation and reduced parasitic capacitance that make commercial devices possible. SiC substrates are available from Cree, Inc., of Durham, N.C. and methods for producing them are set forth in the scientific literature as well as in U.S. Pat. No. Re. 34,861; U.S. Pat. No. 4,946,547; and U.S. Pat. No. 5,200,022.
The efficient extraction of light from LEDs is a major concern in the fabrication of high efficiency LEDs. The external quantum efficiency may be limited by total internal reflection (TIR) of light at a surface of the LED die. TIR can be caused by the large difference in the refractive index between the LED's semiconductor and surrounding ambient. LEDs with SiC substrates tend to have relatively low light extraction efficiencies because the high index of refraction of SiC (approximately 2.7) compared to the index of refraction for the surrounding material, such as epoxy (approximately 1.5). As light in such diodes tends to be emitted through the substrate, this difference results in a small escape cone from which light rays from the active area can transmit from the SiC substrate into the epoxy and ultimately escape from the LED package.
However, even if the substrate is removed, efficient light extraction from an LED die is still a concern.